1. Field of the Invention
The present invention relates generally to a polyvinyl alcohol based polymer and a method of manufacturing the same. More particularly, the present invention relates to a porous polyvinyl alcohol based polymer having low solubility in low-temperature water and a method of manufacturing a polyvinyl alcohol based polymer by allowing a polyvinyl ester based polymer to be subjected to a transesterification reaction (hereinafter also referred to as a “saponification reaction”) with alcohol in the presence of a basic compound under specific conditions.
2. Related Background Art
A polyvinyl alcohol based polymer (hereinafter abbreviated as a “PVA” in some cases) that is a typical water-soluble polymer has excellent strength characteristics and film formation ability in addition to the water-solubility. Using such characteristics efficiently, the polyvinyl alcohol based polymer is utilized widely, for example, as a raw material for common vinylon fibers or high strength vinylon fibers, or for fiber processing materials, coating materials for paper, addition agents for paper, adhesives, emulsion stabilizers, films, butyral resins, cosmetics, medical supplies, and ceramic binders.
A PVA with properties of having low solubility in low-temperature water and quick solubility in high-temperature water may be needed depending on the intended use thereof.
Generally, the PVA is used in the form of an aqueous solution. However, when a substrate such as, for instance, paper is to be coated under a condition of a high shear rate, there are problems that, for example, the coating liquid has an increased viscosity to form stripes or is scattered. Hence, it has been studied to make the size of PVA grains smaller to use them in the form of an aqueous slurry. However, when an inorganic substance such as cement is molded by a sheet forming method, the use of the PVA in the form of an aqueous slurry causes problems that, for instance, part of the PVA is eluted into water to decrease the yield thereof in the molded article or the PVA flows out into waste water. Accordingly, there are demands for a PVA having low solubility in low-temperature water. In addition, when the PVA is dissolved in high-temperature water using a continuous dissolution unit such as a jet cooker, it is necessary to dissolve a large amount of PVA in water per unit time and thus there are demands for a PVA having quick solubility. From the viewpoints described above, there are intense demands for a PVA having properties of tending not to form undissolved lumps due to its low solubility in low-temperature water and dissolving in high-temperature water quickly.
Generally, a PVA is manufactured through the transesterification reaction of a polyvinyl ester based polymer (hereinafter abbreviated as a “PVEs” in some cases) with alcohol in the presence of a basic catalyst. Some conventionally known methods of manufacturing a PVA are described as follows.
U.S. Pat. No. 2,642,419 discloses a PVA powder manufacturing method (a so-called belt saponification method) including supplying a mixture consisting of a methanol solution of PVEs whose concentration is 24 wt % to 40 wt % and a methanol solution of sodium hydroxide on a belt conveyer continuously and then pulverizing and drying a gel substance obtained through a methanolysis reaction. The PVA powder obtained by this method dissolves even in low-temperature water. Hence, only the surface of the PVA powder dissolves and they aggregate to be agglomerated. The PVA powder therefore is not suitable for being dissolved in water using, for instance, a continuous dissolution unit. In addition, the PVA powder obtained by this method contain considerable amounts of methanol used in manufacturing it and nonvolatile compounds such as sodium acetate and volatile organic compounds such as carboxylate that are generated through the methanolysis reaction. When a large amount of volatile organic compounds are contained in the PVA powder, the work environment in which the PVA powder is handled deteriorates and further the PVA aqueous solution is required to be subjected to a wastewater treatment. In the case where the PVA powder contains nonvolatile compounds such as sodium acetate, when the PVA powder is used for electrical parts, electronic components, ceramic binders, etc., there is a possibility that some problems such as insulation failure may arise.
JP40(1965)-3700B discloses a PVA powder manufacturing method in which while a PVEs is transesterified with alcohol, a methanol solution of PVEs is supplied intermittently to the transesterification reaction system. When the PVA powder obtained by this method is intended to be dissolved in warm or hot water, undissolved lumps thereof are generated and a uniform aqueous solution therefore cannot be obtained. Hence, this method employs a process in which after the PVA powder is put into low-temperature water, the temperature of the water is raised gradually to allow the PVA powder to dissolve in the water. Accordingly, there is a problem that it takes a long time to dissolve the PVA powder. In addition, since this PVA powder includes a high ratio of fine powder, there is a problem that the fine powder tends to scatter at the time of opening the seal.
JP45(1970)-33191B discloses a PVA powder manufacturing method in which when a PVEs is transesterified with alcohol, a methanol solution of PVEs is supplied continuously at a rate that prevents the concentration of soluble polymers contained in a reaction mixture from exceeding 1 wt %, and a PVA slurry is collected continuously from the reaction mixture. The PVA powder obtained by this method also has the same problems as those described with respect to JP40(1965)-3700B.
JP46(1971)-9826B discloses a PVA powder manufacturing method in which while a methanol solution including a saponification catalyst and a methyl acetate/methanol mixed solvent of partially saponified PVA whose saponification degree is 10 mol % to 40 mol % is supplied continuously to a slurry of a methyl acetate/methanol mixed solvent of PVA whose saponification degree is 97 mol % to 98.5 mol %, at a rate that prevents the concentration of soluble polymers contained in the reaction mixture from exceeding 1 wt %, a PVA slurry is collected continuously from the reaction mixture and is deliquored and dried. The PVA powder obtained by this method also has the same problems as those described with respect to JP40(1965)-3700B.
There are some PVA powder manufacturing methods proposed for solving the problems of undissolved lumps or agglomeration that are caused in dissolving PVA powder in water.
JP54(1979)-7311B discloses a method of manufacturing PVA powder whose solubility in cold water is decreased by heating a PVA with a saponification degree of 93 mol % to 100 mol % in a mixed solvent including methanol, water, etc. at a temperature of at least 50° C. The PVA powder thus obtained, however, may not have improved solubility in high-temperature water. Furthermore, this method employs complicated processes since the PVA is manufactured first and then is heat-treated, and therefore this method requires an additional apparatus for the heat-treatment. Moreover, energy efficiency is low in this method.
JP2002-53616A discloses a method of manufacturing a PVA having improved solubility in water by introducing a specific azo polymerization initiator into a polymer system of vinyl acetate through a supply line while maintaining the azo polymerization initiator at a low temperature, and saponifying polyvinyl acetate obtained through the polymerization of the vinyl acetate. This method allows a PVA with high crystallinity to be obtained but cannot provide a PVA that tends not to dissolve at low temperatures but dissolves at high temperatures quickly. Furthermore, it is difficult to reduce the amount of volatile organic compounds contained in the PVA by drying. When the volatile organic compounds remain in the PVA, the work environment deteriorates. Moreover, this method requires temperature management to be conducted well to keep the line for supplying the polymerization catalyst at low temperature.
JP2000-265026A discloses PVA powder that contains at least 20 wt % of grains whose diameters are in the range of 500 μm to 1000 μm and is excellent in solubility. The PVA powder proposed in JP2000-265026A has a property of tending not to form undissolved lumps when being dissolved in low-temperature water. It, however, takes a long time to dissolve the PVA powder due to the large grain diameters. Thus the PVA powder lacks in practicability. JP8(1996)-301936A discloses a method of heating a PVA, which has been obtained by a well-known method, at a temperature of 140° C. for two hours using no solvent. According to this method, the temperature dependency of the solution property of the PVA powder increases. As in the case of JP54(1979)-7311B, this method, however, employs complicated processes since the PVA that already has been manufactured is heat-treated, and thereby requires an additional apparatus for the heat-treatment. Moreover, energy efficiency is low in this method.
Furthermore, methods of manufacturing a PVA with improved solubility in water also have been proposed.
JP9(1997)-316272A discloses a method of manufacturing porous PVA powder by saponifying a mixed solution including a PVEs and a partially saponified PVA having a saponification degree of 20 mol % to 60 mol %. The PVA powder obtained by this method has been improved in having decreased solubility in low-temperature water. The PVA powder, however, still has room for improvement in solubility in water whose temperature has been raised. In addition, since this method employs the partially saponified PVA, this method requires a preliminary process of saponifying a PVEs.
JP8(1996)-188619A discloses a method of manufacturing PVA fine grains by saponifying a PVEs, with the PVEs being dispersed in a dispersion medium such as a liquid paraffin that does not allow any of the PVEs, PVA, and alcohol to dissolve therein. The PVA fine grains obtained by this method have been improved in having decreased solubility in low-temperature water. There, however, is a problem in that it is difficult to remove volatile organic compounds contained in the PVA fine grains since they are not porous. Furthermore, this method requires the use of a dispersant such as polyvinylpyrrolidone. In order to remove such a dispersant from the PVA, complicated processes are required.
There are intense demands for the PVA that has properties of tending not to form undissolved lumps due to its low solubility in low-temperature water and dissolving in high-temperature water quickly. In the past, a PVA with decreased solubility in low-temperature water has been proposed, but no PVA whose solubility in water varies with a small temperature change to a sufficiently great degree has been obtained yet. Furthermore, conventionally, additional processes or operations are required to suppress the solubility in low-temperature water.